Implement rpc server method: sendrawtransaction (#174)

* Added new config attributes: 'SecondsPerBlock','LowPriorityThreshold'

* Added new files:

* Added new method: CompareTo

* Fixed empty Slice case

* Added new methods: LessThan, GreaterThan, Equal, CompareTo

* Added new method: InputIntersection

* Added MaxTransactionSize, GroupOutputByAssetID

* Added ned method: ScriptHash

* Added new method: IsDoubleSpend

* Refactor blockchainer, Added Feer interface, Verify and GetMemPool method

* 1) Added MemPool
2) Added new methods to satisfy the blockchainer interface: IsLowPriority, Verify, GetMemPool

* Added new methods: RelayTxn, RelayDirectly

* Fixed tests

* Implemented RPC server method sendrawtransaction

* Refactor getrawtransaction, sendrawtransaction in separate methods

* Moved 'secondsPerBlock' to config file

* Implemented Kim suggestions:
1) Fixed data race issues
2) refactor Verify method
3) Get rid of unused InputIntersection  method due to refactoring Verify method
4) Fixed bug in https://github.com/CityOfZion/neo-go/pull/174#discussion_r264108135
5) minor simplications of the code

* Fixed minor issues related to

1) space
2) getter methods do not need pointer on the receiver
3) error message
4) refactoring  CompareTo method in uint256.go

* Fixed small issues

* Use sync.RWMutex instead of sync.Mutex

* Refined (R)Lock/(R)Unlock

* return error instead of bool in Verify methods

pkg/core/blockchainer.go

@@ -25,7 +25,7 @@ type Blockchainer interface {
 	GetAccountState(util.Uint160) *AccountState
 	GetTransaction(util.Uint256) (*transaction.Transaction, uint32, error)
 	References(t *transaction.Transaction) map[util.Uint256]*transaction.Output
-	FeePerByte(t *transaction.Transaction) util.Fixed8
-	SystemFee(t *transaction.Transaction) util.Fixed8
-	NetworkFee(t *transaction.Transaction) util.Fixed8
+	Feer // fee interface
+	Verify(t *transaction.Transaction) error
+	GetMemPool() MemPool
 }